| Efficient nitrogen removal can be achieved during simultaneous nitrification and denitrification(SND)process in the biocathode of microbial fuel cell(MFC)system,but the nitrogen removal efficiency needs to be further strengthened.Studies have shown that the three-anode MFC(3A-MFC)has better power generation and pollutant removal performances than the traditional dual-chamber MFC.In this paper,the microbial and kinetic aspects of the method to enhance the nitrogen removal efficiency during 3 A-MFC cathodic SND process were studied.After sampling and screening several strains of nitrifying bacteria and denitrifers from the three-anode MFC cathode chamber that realized cathodic SND,and the effect of weak electric field on their nitrification or denitrification process was studied.A continuous-flow 3A-MFC electrochemical model and a cathodic SND model were developed to analyze the methods to enhance the power generation performance and the nitrogen removal efficiency of the 3 A-MFC.Cathode electron utilization of an intermittent flow sealed MFC experiment was conducted and a model describing the electron transfer pathways in the liquid and gas phases of the cathodic SND process was developed.The electron utilization efficiency of cathodic SND process was evaluated and the methods to improve the cathodic SND process were proposed through simulation analysis.The main conclusions are as follows:(1)Five strains of ammonia-oxidizing bacteria(AOB),six strains of nitrite-oxidizing bacteria(NOB),and four strains of denitrifier were obtained by a sieve experiment after sampling from a 3A-MFC cathode chamber that realized cathodic SND,and AOB strain All,NOB strain N16,and denitrifier strains D6 and D4 had stronger activity among similar strains.Weak electric field stimulation experiment results showed that when the constant potentials of 0 V,0.2 V,and 0.4 V vs the saturated calomel electrode(SCE)were applied,the adaptation period of AOB strain All was shortened by 2 days compared with the control,but its NH3-N removal rate was not promoted;the nitrite oxidation and growth of NOB strain N16 were not significantly promoted compared with the control.When the constant potentials of 0 V,-0.2 V,and-0.4 V vs SCE were applied,the denitrification and growth rates of denitrifier strain D6 were not significantly promoted compared with the control.When the constant currents of 2 mA,10 mA and 50 mA were applied,the denitrification and growth of denitrifier strains D6 and D4 were both inhibited.When applying a smaller constant current(0.1 mA),the denitrification and growth processes of the D6 and D4 strains were not significantly promoted.(2)The electrochemical model described the relationship between the output voltage of the 3A-MFC and the loss of overpotential.The cathode overpotential was related to the autotrophic denitrification process of the cathode.Based on the Activated Sludge Model No.1(ASM1),a 3 A-MFC cathodic SND model and a cathodic SND electron transfer pathway model were established,which not only simulated and predicted the conventional autotrophic nitrification,heterotrophic denitrification and organic heterotrophic oxidation in the cathode chamber,but also described the autotrophic denitrification and the reaction of oxygen with the electrons transferred from the external circuit.(3)The electrochemical model simulation results showed that the MFC overpotential losses were mainly the ohmic overpotential(ηohmic)and the activation overpotential(ηact),and their proportions were equal.The cathode activation overpotential(ηc)accounted for most of the total activation overpotential.The 3A-MFC cathode model could well simulate the variation trends of NH3-N,NO3--N and chemical oxygen demand(COD)concentration during the cathodic SND process.The determination coefficients(R2)all exceeded 0.995.(4)The experiment changing the concentrations of electron donors(NH3-N,COD)and electron acceptor(O2)in the cathodic influent of the dual-chamber MFC showed that the dual-chamber MFC achieved COD and total nitrogen(TN)removal efficiencies up to 88.7±9.9%and 87.3±0.8%,respectively.The MFC cathodic SND efficiency was higher than 68%,and the MFC achieved a power output of up to 2.96 W m-3.The increase in the amounts of electron transferred from the external circuit to the cathode and the C/N ratio was beneficial to the cathode TN removal efficiency increase.The cathode electron utilization efficiency(Ee)could reach nearly 100%,and the cathode electrons tended to be completely utilized with the increase of the autotrophic denitrification ratio in the cathodic denitrification process(Ra).The electron transfer pathways model precisely fit the concentration variations of NH3-N,dissolved oxygen(DO)and COD during the cathodic SND process(R2≥0.987).(5)The simulation results of the 3A-MFC cathodic SND model and the electron transfer pathways model showed that there was a competitive relationship between autotrophic denitrifiers and heterotrophic denitrifiers;the increase in the rate of nitrification was conducive to the acceleration of autotrophic denitrification;heterotrophic denitrifiers contributed more to the cathode nitrogen and carbon removal efficiencies than autotrophic denitrifiers;the measures to improve the TN removal efficiency and SND efficiency of the 3A-MFC cathode were:1)Do not add NO3--N into the cathode influent;2)Properly reduce the concentration of NH3-N in the cathode influent;3)Increase the hydraulic retention time;4)Use electrode materials with high specific surface area to reduce the activation potential loss of the cathode;5)Properly reduce the DO concentration in the cathode under the premise of ensuring the occurrence of SND. |
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